Upper Anisian to Lower Carnian Magnetostratigraphy from the Northern Calcareous Alps (Austria) Yves Gallet, Leopold Krystyn, Jean Besse

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Upper Anisian to Lower Carnian magnetostratigraphy from the Northern Calcareous Alps (Austria) Yves Gallet, Leopold Krystyn, Jean Besse

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Yves Gallet, Leopold Krystyn, Jean Besse. Upper Anisian to Lower Carnian magnetostratigraphy from the Northern Calcareous Alps (Austria). Journal of Geophysical Research : Solid Earth, American Geophysical Union, 1998, 103 (B1), pp.605-621. insu-01863517

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HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNALOF GEOPHYSiC• RESEARCH,VOL 103,NO. B1, PAGES 605-621, JANUARY 10, 1998

UpperAnisian to Lower Carnianmagnetostratigraphy from the Northern CalcareousAlps (Austria)

Yves Gallet Laboratoirede Pa16omagn6tisme, URA 729, Institutde Physique du Globede Paris, France

LeopoldKrystyn Institutefor Paleontology,Vienna, Austria

Jean Besse Laboratoirede Pa16omagn6tisme, URA 729, Institutde Physiquedu Globede Paris, France

Abstract. We presenta magnetostratigraphicstudy of threepelagic limestone sections (Gamsstein,Mendlingbach and Mayerling) from the eastern part of theNorthern Calcareous Alps.Together these sections, which contain a richconodont fauna, yield a sedimentaryrecord encompassingthe uppermost Anisian, the Ladinian and the Lower Carnian. Thermal demagnetizationand isothermal remanent magnetization experiments indicate that the magnetizationis essentiallycarried by a mineralof themagnetite family. The high unblockingtemperature components isolated from thethree sections provide clear magnetic polarityzonations. Correlations between these results, based on the biostratigraphic data and tephrochronology,allow the construction of a compositemagnetic polarity sequence from the Illyriansubstage (Upper Anisian) to theJulian 2 zone(Lower Carnian). This sequence contains17 well-definedpolarity reversals, and eight more poorly defined intervals. Correlationscan be suggestedbetween the new dataand other magnetostratigraphic results previouslyobtained from marine sections. We estimatethat the magnetic reversal frequency was2.5 to 3.6 reversalsper million yearsduring the Ladinian.

Introduction reliable compositemagnetic polarity sequencefrom these latter resultsis problematic,they shouldbe constrainedby additionalmagnetostratigraphic data from well-datedmarine The Triassic magnetostratigraphy has been recently sections. improved by several studies both on marine and continental In this study we report the magnetostratigraphyof three sedimentarysections [e.g., Steiner et aI., 1989; Ogg and pelagiclimestone sections from the NorthernCalcareous Alps Steiner, 1991; Gallet et aI., 1992, 1993a, 1994, 1996; (NCA; Austria). Correlationsbetween these sections based on Muttoni and Kent, 1994; Muttoni et al., 1994, 1995, 1996, conodontsand taphrostratigraphyallow the constructionof a 1997; Kent et al., 1995; Molina-Garza et al., 1991, !993, magneticpolarity sequencefrom the UppermostAnisian to 1996]. However, these data concentrateon the magnetic the Lower Carnian. reversalchronology during the Late and Early Triassic.For the Middle Triassic, magnetostratigraphicresults obtained from marine sedimentsare available only for the Lower and Middle Anisian [Muttoni et al., 1995, 1996] and from the Ladinian Biochronologyand Chronostratigraphy UppermostAnisian to the Lower Ladinian[Muttoni and Kent, !994; Muttoni et al., 1994]. Other studies that provide Several recent studies that focused on the Ladinian informationabout Middle Triassicmagnetostratigraphy come biochronology[e.g., Brack and Rieber, 1993, !994; Kovacs from the southwestern United States (upper Moenkopi et al., 1994; Mietto and Manfrin, 1995] show differences formation[e.g., Helsley and Steiner,1974; Baag and Helsley, both in the content and scope of the recognized 1974;Elston and Purucker,1979; Lienert and Helsley, 1980; biostratigraphiczones. Consequently,the Ladinian may Puruckeret al., 1980; Molina-Garza et al., 1991, 1996]) and either contain six ammonoid zones (with the inclusion of the from Spain[Turner et al., 1989]. Thesedata were obtained Reitzi zone [Kozur, !995]), five zones (beginningwith the from continentalsediments (red beds)where biostratigraphic Nevaditeszone [Krystyn, 1983]) or four zones (beginning controlis generallypoor. And becausethe constructionof a with the Curionii zone [Brack and Rieber, 1994; Bucher and Orchard, 1995]). In this study we will use the secondvariant which correspondsto possibilityB suggestedby Brack and Copyright1998 by the American Geophysical Union. Rieber(1994). In this way, the baseof the Ladinianis marked Papernumber 97JB02155. by the onset of the Nevadites secedensiszone and 0!48.0227/98/97JB.02155509.00 approximatesclosely to the appearanceof the conodont

605 606 GALLET ET AL.: MAGNETOSTRATIGRAPHY FROM NORTHERN CALCAREOUS ALPS

Paragondolellatram/neri [see Krystytz, 1983; Nicora and modified by introducing the Secedensis zone, and also Brack, 1995; Muttoni et al., 1997]. The Lower Ladinian stratigraphically by shifting beds 10 and 11 from the contains two ammonoid zones (the Fassanian1 and 2 zones) Archelauszone downwards to the Gredlerizone (Figure 1). The andthe Upper Ladinian three zones (the Longobardian 1 to 3 reasonfor this revisionis the discoveryof a Protrachyceras zones). gredleri specimenin bed 10. Bed 11 lacks diagnostic The three Ladinian sectionsanalyzed in this study come ammonoids but it has identical lithology and microfaciesas from the Reifling formationwhich is apparentlydevoid of the underlying levels 10 and 9. The other changesin ammonoids but contains rich conodont faunas. The latter have conodont faunas concern Paragondolella excelsa (Last been grouped into six intervals and have been OccurrenceDatum [LOD] in bed 6/3), P. inclinata (First chronostratigraphicallyfixed by conodont-basedcorrelations Occurrence Datum [FOD] in bed 7), the introduction of with the ammonoid-rich sections of Epidaurus (Greece) and Budurovignathus longobardicus from beds 13 to 17 andthe Bagolino(Northern Italy). However,the chronostratigraphy removal of Gondolalla cf bakalovi (see commentof Kovacs of the Epidaurussection needs to be reconsideredbecause [1994]). Within the Upper Fassanian,the former Hungatica additional ammonoid and conodont faunal data collected in zonehas been renamed for P. excelsain orderto avoidany 1994 provided new stratigraphic constraints (Figure 1) furtherconfusion with the originalscope of this zone[Kozur, [Krystyn, 1983]. The ammonoid zonation is nomina!y •980].

FASSANIAN LONGOBARDIAN • •

ß • , • • • , •" z z •' • ' • • i • m o

I • • • ½ • • m m • • % • • • • • • g • bed hr. I I• I ., i..... I .....III I I , 1"1

tramtoefl excelsa incl. mung. Iongobard. diebell

......

• P, trammeri •i I

P, excelsa • •

P. Iiebermanni • P. inclinata --

. • B. /apontcus• B, Iongobardicus I i i • • •B. mungoensis I I ' II IIIIiIII II II I I I I • • • B. diebeli

I I ! • • • B. mostleri

I I i ! ...... I ,, I ...... I ,, ! ....

P. excelsa !• I• M...... t=dp ole

N. pseudoionga • P. inclinata

I ! i • B. hungaricus • N. transira • • B. m ungoensis m

I• N. praehungarica• B. Iongobardicus • I • • Z

i• •i i• B. diebell

i P, trammeri i mostleri

iI I! iI G. malayensis I I I I I I IIIIIII II I I ! I I ß ,,,,i ...... I ,,, i ......

pseud, transita incl. mung. Iongobard. diebeli

......

Figure 1. Ladinian biochronologyused in this study. GALLlETET AL.: MAGNETOSTRaTIGRAPHYFROM NORTHERN CALCAREOUS ALPS 607

Many sectionsof the Reifiing basincontain three distinct three major tuffaceous intervals which are located in the well- tuffaceouslayers with a light grey to greenishprimary and known Bagolinosection at 61 m, between70 and 75 m and 84- .,,ellowishweathering secondary color. These layers probably 88 m, respectively[Bracl, et al., 1995]. Accordingto Nicora representaltered pietra verde. They have been numbered as T1, and Brack [1995], these intervals fit closely in age •ith the T2 and T3 in our sectionsand are interpretedas geologically three Reifling basin layers. isochronousand therefore chronostratigraphic marker beds. Tephrochronologythus provides an independenttool to constrainthe adopted correlationsbased on conodontsand to GeologicalSetting, Lithology and Sampling establish long-distance time relationships with other tuff- bearingLadinian sections of the Alpine region.In particular, Kozttr and Mostlet' [1971] described a section from the The studied sections are located in the eastern part of the Nemesvamos formation (Koeveskal, Balaton Highland, Northern Calcareous Alps, within the so-called Bajuvaricum Hungary)with two distincttuffaceous intervals at 7.5 and 11.5 mega-unit. This unit consists of a series of north vergent m which may correspond to the T2 and T3 layers of the nappesstacked between the Cretaceousand the early Cenozoic Reifling basinaccording to the conodontdata. The T2 and T3 [e.g., Tollmann, 1976]. Two of the sampling sites, the layersmay be also identifiablein Reifiinglimestones from the Mendlingbachand Gamssteinsections, are situatedin a local Western Calcareous Alps (e.g., from the Martinswand section tectonic structure, the "Reifiinger Scholle", intercalated from Tyrol [Bechsttidt and Mostlet', 1974]). betweenthe Lunz nappeto the north and the Oetschernappe to Tephrochronologiccorrelations with the Buchensteinbasin of the south. This structure, which has a maximum length of northernItaly are more difficult becauseof the large numberof approximately10 km betweenthe village of G6stling and the tuffaceouslayers in this basin. However, groupedon the basis Enns valley (Figure 2), is itself divided into smaller units of local frequencypeaks, the Buchensteinsequence also shows called "Schuppen"of which the Gamsstein-Schuppeincludes

ß?.:'-....' ::-

UTR •• LunzE- ..::::•:...... ::.:....,.. .:). : " L_LJ Wetterstein L..':';' ß

MTR • ReiftingE":...... '"'?. • Steinaim L.• .' :.

500 m

GAMSSTEIN S.

14050 '

47ø45

Lassing

Vienna

Salzburg

Palfau _. Innsbruck

Figure2. Locationmap of theGainsstain (B) andMendlingbach (A) sectionsand simplified geologic map of the samplingarea 608 GALLETET AL.'MAGNETOSTRATIGRAPHY FROM NORTHERN CALCAREOUS ALPS

• Ze • I,

z GALLETET AL.:MAGNETOSTRATIGRAPHY FROM NORTHERN CALCAREOUS ALPS 609

1.0

0.6 0.4

0.2

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

B (Tesla)

1.0

0.8

b) O.6 0.4

0.2

0 0 0.! 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.I 1.2

B (Tesla)

Figure 4. IRM analysesof samplesfrom the (a) Gamssteinand (b) Mendlingbachsections. In both sectionsthe saturationis almostachieved in a 0.3 T field which likely indicatesa magnetizationdominated by a mineralin the magnetitefamily. A smallfraction of a high-coercivitymineral is alsoobserved. the two sections(Figure 2). The third section(Mayerling) is relatively abundant conodonts. The intercalations of locatedabout 100 km to the east of the other two sections, redepositedshallow-water debris in Mayerlingand the close approximately30 km southwest of Vienna (see Gal!et et al., proximity of platform carbonatesin Gamssteinand 1994,Figure 1]. A previousstudy of thissection provided a Mendlingbachsuggest a specificoceanographic environment LowerCamian magnetostratigraphic sequence [Gallet et al., for these sections. 19941. Ladinianstrata crop out widely within the NCA. Theycan be Gamsstein Section broadlygrouped in two mainsedimentary environments: (1) largeshallow-water carbonate platforms of the Wetterstein This sectionis located about 1 km northeastof the village formationand (2) deeperbasins filled with hemipelagic of Palfau,and approximately 2 km southwestof Mendlingbach limestonesof the Reifling formation.The latter usually (Figure2, siteB). The sectionoutcrops at an altitudeof !010 consistsof dark grey, chert-rich,thin-bedded filament and m, alonga forestroad leading from the Bundesstrasse 25 to the radiolarian-bearingwackestones. Although belonging to the Gamssteinplateau, at the westernside of the deeplyincised Reiflingformation, our sectionsshow some facial differences Raftel grabben.The sectionwas dividedinto two subsections as they are composedof light colored(grey to yellowish) about 25 m from each other. The first site (Gamsstein 1) limestonesof various thicknesseswith less chert. The consistsof 27 m of UpperLadinian sediments from whichwe microfaunais also more diverse, containing foraminifera and collected 88 palcomagneticsamples. Twenty five samples 610 GALLETET AL.:MAGNETOSTRATIGRAPHY FROM NORTHERNCALCAREOUS ALPS

o of the samevalley wherea forestroad was recentlybuilt. This latter section is important becauseit includes a 13-m-thick intervalthat is missingin Mendlingbach-Westdueto faulting. a)Gamsstein Fifty five conodontsamples were collected from the whole sequence,and the relateddata indicate an age from the Lowerto the Upper Ladinian. Over 100 paleomagneticsamples were drilled in the Mendlingbach-East section and 40 in the Mendlingbach-West section.

: .-':: 1 Mayerling Section 270[++•+ + + + +++ + + This section is exposed along a railway cutting, 1 km northwestof the village of Weissenbach(see Gallet et al., o 0 1994,Figure 1 ]. A newstratigraphic interval of approximately 40 m was sampled.This sequencediffers lithologicallyfrom typical Reifling limestonesbecause of thicker bedding,lack of chert bands, and by the presenceof stromatactis-!ikecalcite cavitiesup to 5 cm in length and 2 cm in height. Twentwfive conodont samples provided a faunal record from the Longobardian 2 to the Longobardian 3 zones (Upper Ladinian). Forty sampleswere collected for paleomagnetic analysis.

270 90 Chronostratigraphy and Biostratigraphy of the Reifling Sections

The Upper Anisian (Illyrian substage) is very thin in the investigatedsections (approximately 1.5 m) and startswith a distinct erosional surface. Accumulationsof cephalopodshell fragments and the microfacies (echinoderm and glauconite bearing packstones) indicate bottom current activity with possible erosion and/or nondeposition of sediments.The 270 90 upper half of this stratigraphic interval is dated by ammonoids. Aplococeras ? sp., "Ceratites" cf. lenis, Parakellnerites rothpIetzi, P. zonianensis,Hungarites ? cf. plicatus, Flexoptychites acutus, F. noricus, F. gibbusand Ptychites arthaberi were found in Gamsstein (bed M1). Following Brack and Rieber [1994], this fauna clearly characterizesthe Reitzi zone and is probably located closeto the base of the Nevadites recte Secedensis zone sensu Brack and Rieber [1994]. The rich conodontfauna of M1 includes Neogondolellaconstricta./cornuta, Paragondolella liebermanni and P. excelsa.The FOD of N. pseudoIongais locatedthree 180 beds above M1 (about 60 cm), less than 10 cm below the Figure 5. Equal-area projection of directionsof the high- tuffaceouslayer T1. For practicalreason, the Anisian-Ladinian unblocking temperature component obtained from the (a) boundaryis drawnhere at thebase of T1, 20 cmbelow the FOD Gainsstein 1 and (b) Gamsstein2 subsections.Solid (open) of P. trammeri.Following Nicora and Brack [1995] whohave symbolsrefer to directionsin the lower (upper)hemisphere. found N. transira from the Chiesense level (top of the (c) The meannormal and reversed polarity directions. Secedensiszone) of Bagolino,the Fassanian1 to 2 boundary hasto be placedwithin the Transitazone (Figure 1). The boundarybetween the Lower and Upper Ladinian were drilled from the second (Gamsstein 2), which is coincides with the FOD of P. inclinata as documentedfrom stratigraphicallythinner (5 m) with an age rangingfrom the ammonoid-datedrocks from 'Epidaurus.Within the Upper UpperAnisian to the LowermostLadinian. Approximately 50 Ladinian,no corresponding boundaries between ammonoid and conodontsamples were obtainedfrom thesetwo subsections. conodont zones could be established. The adopted Mendlingbach Sections Longobardian1/2 boundaryresults from a tephrochronologic correlationof the Reifling T3 layer with the 84-88 m Two sectionswere sampled near the small village of Lassing tuffaceousinterval of the Bagolinosection. The latterhorizon (Figure2). The first, calledMendlingbach-East, is exposed is includedinto the lower Longobardian 2 zone on the basis of alonga roadat the easternside of a valley,about 1 km northof ammonoidsand daonellids by Bracketal. [1995].Conodonts Lassing.It has a thicknessof about 30 m. The second,called support the above postulated correlation because Mendlingbach-Westsection, is locatedalong the westernside Budurovignathusmungoensis is observed in thesetuffaceous GAtJ•ET ET AL.:MAGNETOSTRATIGRAPHY FROM NORTHERN CALCAREOUS ALPS 611

levels from both sections. The boundary between Longobardian2 and 3 cannot be well constrainedby conodontsand it mightbe locatedsomewhere in the upperpart of the Longobardicuszone. Finally, the Ladinian/Carnian boundary is datable by the FOD of Paragondolella polygnathiformis, a datum already used in our earlier magnetostratigraphicstudies [Gallet et al., 1993a, 1994]. The rangesof platformconodont species in the Gamsstein, Mendlingbachand Mayerlingsections are shownin Figure !. N. pseudolonga,N. transita, P. excelsa,P. trammeri and Gladigondolellatethydis are the mostabundant species found in the LowerLadinian, and P. inclinata,B. mungoensisand G. malayensisare the mostabundant species found in the Upper Ladinian. We paid special attention to the fact that some stratigraphicguide forms have more limited ranges in the Reifiing intraplatform basin than in the eupelagic to miopelagic settingsof Epidaurusor of the Balaton region [Kovacs,1994]. Indeed,some species common in the Reifling basin(e.g., N. pseudolonga,N. transitaand Metapolygnathus tadpole) are missing in Epidaurus. Some others (N. praehungarica, P. trammeri and B. hungaricus) have more limited ranges in the Reifiing limestones. Therefore the detailedvertical distributionof all specieshas been primarily establishedand usedfor correlationof the sectionssampled for magnetostratigraphy. Future studies may decide on its applicability as a local reference scale for Ladinian intraplatformbasins of the Alpine region. The discriminatedzones are all definedby the FOD of their nominate speciesexcept for the Diebeli zone. In our sections, N. pseudolongareplaces N. cornuta or N. constricta cornuta sensuKovacs [1994] in the bedjust belowT1. P. trammeriused by Krystyn [1983] as a marker for defining the base of the Ladinian in the pelagic facies, appearsdirectly below T1 in the Mendlingbach-Eastsection and slightly higher (0.2 m) in the Gamsstein section with rare juvenile individuals but becomes common above. P. excelsaand G. tethydisrange throughout the Pseudolongazone. The suddenincrease in frequencyof the latter species could be used as an additional lower zonal boundaryindicator. N. transita which may be a synonymof N. excentrica (see also comments by Kovacs [1994]) occurs together with G. tethydis, P. excelsa and P. trammeri throughoutthe Transitazone. B. hungaricusappears with rare specimensin the upper half of this zone. Within the !nclinata zone, P. inc!inata,G. tethydis,B. hungaricusand P. trammeri are common.In contrast,N. praehungarica is relatively rare and restricted to the Gamsstein section. A single but diagnosticspecimen of B. japonicus was helpful to define the Fassanian-Longobardianboundary within the Paragondo!ella- free interval of the Mendlingbach-West section. B. mungoensis, G. tethydis,P. trammeri, P. inclinata and G. ma!ayensis range throughout the Mungoensis zone. B. hungaricus, which is long ranging in the Epidaurussection, is restricted to the lower half of this zone. The index form of the Longobardicus zone is common in Mendlingbach but relatively rare in the Gamssteinand Mayerling sections.G. malayensis,P. inclinata andP. mungoensisrange throughout this zone in all sections.P. fueloepi (sensuholotype Kovacs [1994]) and P. trammeri occur sporadicallyin Gamssteinand Mayerling, the latter of which disappearswithin the zone.The FOD of Metapolygnathus tadpole occurs low within the Longobardicuszone in the Mendlingbach-Eastsection. As B. diebeli has a very long range, here.we define the base of the 612 GALLETET AL.: MAGNEtOSTRATIGRAPHYFROM NORTHERNCALCAREOUS ALPS

Diebeli zone by the FOD of B. mostleri. This boundary is PaleomagneticResults recognizedonly in the Mayerling section,as time equivalents have not been sampledin the other sections.Throughout the Palcomagneticanalyses were carried out with a three-axis Diebell zone, the faunal compositionis quite uniform and CTF SQUIDmagnetometer in the magnetically shielded room dominatedby P. inclinata, G. malayensisand B. mostleri. B. atthe Institut de Physique duGlobe. Samples were thermally mungoensis and B. diebeli are less frequent and B. demagnetizedin approximately 15steps using a laboratory- !ongobardicus is presentonly in the lowermostpart of this built furnace.For comparison,several samples were also zone. Althoughmissing in the Mayerling section,M. tadpole demagnetizedby alternatingfields (AF) with a GSD1 is known from many time-equivalentsections of the Reifling Sch6nstedtdemagnetizer. The paleomagneticdirections were basin.The upper limit of the Diebeli zone has been modified determinedby leastsquares analysis [Kirschvink, 1980], and by Krystyn [1983] to coincide with the FOD of the statisticalmethods of Fisher [1953] andMcFadden and Metapolygnathuspolygnathiformis. McElhinny[1988] were used to computethe mean directions.

Declination Inclination Magnetic a) Age Conodonts 0 90 180 270 360-90 -60 -30 0 30 60 90 zonation i

Declination Inclination b) Age Conodonts 0 90 180 270 360-90 -60 -30 0 30 60 90

I ! , I I

, I I I ..... I

Figure6, Magneticstratigraphy obtained from the (a) Gamsstein1 and (b) 2 subsections.The age zonationis establishedfrom con0donts. T1, T2 andT3 indicatethe location of threetuffaceous layers. A sequenceof 11 magnetic polarity intervals is obtained from Gainsstein 1,one of thembeing poorly defined by a singlesample in thetopmost part of thesubsection. Another poorly defined magnetic interval is observedduring the Illyrian stage from the Gainsstein 2 subsection. GAIJ ._R7F ETAL.: MAGNETOSTRATIGRAPHY FROMNORTHERN CALCAREOUS ALPS 613

Gamsstein and Mendlingbach Sections componenthas clearly a reversedmagnetic polarity before beddingcorrection. Finally, a highunblocking temperature (or Except for the samplesfrom the upper part of the characteristic)component with the two polarity statesis Mendlingbach-Eastsection, the bulk susceptibilitymeasured isolated between about 500øC and 580ø-600øC (normal after each demagnetizationstep remainsstable during polarity,Figures 3a, 3e and 3h; reversedpolarity, Figures 3b, progressivethermal demagnetization. This indicatesthat no 3c, 3d and3g). . noticeablechange in magneticmineralogy occurs during the The magneticbehavior is more complexin samplesfrom heating-coolingprocess. A similar palcomagneticbehavior is the upper part of the Mendlingbach-Eastsection. That observedboth in Gamssteinand Mendlingbach. Examples of differenceis illustratedby largechanges in bulk susceptibility demagnetizationdiagrams are shown in Figure 3 before above 450øC. This clearly shows the occurrence of beddingcorrection. They indicate the presenceof three mineralogle transformations in these samples, which magneticcomponents. A first componentof normalpolarity sometimes prevents the determination of a reliable high is removedbetween the NRM and approximately200øC. unblockingtemperature component. However, the magnetic Before bedding correction, the directions obtained for this polarity of thesesamples appears unambiguous (Figure 3h). componentare close to the directionof the presentEarth's isothermalremanent magnetization (iRM) experiments magneticfield. An intermediatecomponent is removednext show that most of the magnetizationis saturatedin a 0.3 T between200øC and about 450øC. Although this component is field, but a small fractionof high-coercivityminerals is also presentin many samples (e.g., Figures 3b and 3e), its observed (Figures 4a and 4b). Together with maximum direction can never be precisely determined because of unblockingtemperatures close to 580øC-600øC(Figure 3), significantoverlap of its temperaturespectrum. However, this these characteristicsindicate that the magnetization is

0 0 0

27o 9o

180 180 180

0 0

27O • 9O

180 180

Figure 7. Equal-areaprojections of directionsof the high-unblockingtemperature component isolated throughoutthe (a, b, c) Mendlingbach-Eastand (d, e) Mendlingbach-Westsections. Same conventionas in Figure5. Althougha largescatter is observedin directionsfor the UpperLadinian from the Mendlingbach- Eastsection (Figure 7b), a rotationof approximately20 ø is observedbetwen the Lowerand UpperLadinian from this section(Figure 7c). 614 GALLETE1 • AL.:MAGNETOSTRATIGRAPHY FROM NORTHERN CALCAREOUS ALPS essentiallycarried by a mineralof the magnetitefamily. The smallfraction of high-coercivityminerals may correspondto the presenceof hematiteor iron sulfides.In particular,the presenceof pyrrhotitemay explain the changesin magnetic mineralogyobserved during thermal demagnetizationof samplesfrom the upperpart of theMendlingbach-East section. Further confirmation of high coercivity minerals is also obtainedfrom alternatingfield (AF) demagnetizationup to 70 mT of samplesfrom the Gamssteinsection (Figure 3d). In thesesamples a componentof reversedpolarity before bedding correctionis clearly not demagnetized.We suggestthat this lattercomponent is equivalentto the intermediatecomponent isolatedby thermaldemagnetization. The directions of the high unblocking temperature componentisolated throughout the two Gamssteinsubsections are shown in Figure 5. Both normal and reversed polarity directions are observed. The mean normal and reversed directions are not exactly antipoda! (Table 1; T=10.0 ø, Tc=6.8ø) [McFadden and McEIhinny, 1990]. This is probably due to the presenceof the poorlydefined intermediate magnetic componentwhich may introducea bias in the determinationof the high-unblocking temperature component. Nevertheless a clear magnetic zonation is obtained from both subsections (Figures6a and 6b). Two intervals, probablyof short duration, are defined by single samples, one during the uppermost Longobardian2 zone (Gainsstein1; Figure 6a) and the second duringthe !11yrian(Gamsstein 2; Figure 6b). The directionsisolated from the Mendlingbach sectionsare i i shownin Figure7. Two equalarea projectionsare presentedfor i the Mendlingbach-Eastsection, separatingdata of the lower and upperLadinian, from each side of a fault. Althougha large scatteris observedfor the Upper Ladinian (Figure 7b), which is likely due to the alterationof the magneticmineralogy during thermal demagnetization, two statistically different mean directionsare observed(Figure 7c), showinga relative rotation of 20.2ø +/- 8.1ø betweenthe two parts of this section (Table 2). In contrast, the directions obtained for the Lower Ladinian from the Mendlingbach-Eastsection and from Mendlingbach- West section are better clustered. But as in Gamsstein, the mean normal and reversed polarity directions are not exactly antipodal: Mendlingbach-East: ¾=14.6ø , Tc=10.0ø; Mendlinbach-West: T=5.5 ø, Tc=5.3ø [McFadden and McEIhinny, 1990]. A sequenceof 10 magnetic polarity intervals is obtained from the Mendlingbach-East section (Figure 8a). An interval of normal polarity within the Longobardian1 zone is poorly defined, one sample showing an intermediate direction. The location of the fault at the Lower-Upper Ladinian boundarycoincides with a changein magneticpolarity, confirminga sedimentarygap at this level. The Mendlingbach-Westsection yields a sequenceof eight additional polarity zones, but two of them are again poorly definedby single samples(Figure 8b).

Mayerling Section

The paleomagneticbehavior of thesesamples is identicalto the one previouslyobserved in the Lower Carnianpart of this section [Gallet et al., 1994]. A descriptionof this behavior can thus be found in this previous study. The directions isolatedfor the high unblockingtemperature component are reportedin Figure 9a. The mean directioncomputed for the Upper Ladinian is statistically similar to the direction GAIJ,RT ET AL.: MAGNETOSTRATIGRAPHY FROMNORTHERN CALCAREOUS ALPS 615

Declination Inclination a) Age Conodonts Magnetic 90 180 270 0 90 -90 -60 -30 0 3O 60 90 zonation ' i i ' i"'

o •

AN!S• ,

Declination Inclination Conodonts Magnetic b) Age 90 180 270 0 90 -90 -60 -30 0 30 60 90 zonation

Figure 8. Magnetic stratigraphyobtained from the Mendlingbach-Eastand Mendlingbach-West sections.In both sectionsthe age zonationis establishedfrom conodonts.(a) A magneticsequence of 10 intervalsis obtainedfrom Mendlingbach-East.F indicatesthe locationof a fault betweenthe Lower and UpperLadinian. (b) Eight magneticintervals are obtainedfrom theMend!ingbach-West section, but two of them,during the Longobardian1, are poorlydefined. 616 GAI,! ,ET ET AL.: MAGNETOS•TIGRAPHY FROM NORTHERN CALCAREOUS ALPS

27 90

270 90

180

Figure 9. (a) Equal-areaprojection of the directionsisolated for thecharacteristic component from the Upper Ladinian part of the Mayerlingsection. (b) A comparisonbetween the Upper Ladinian and the Lower Carnian mean normal and reversed polarity directions.

obtainedfor the LowerCarnian (Table 3 andFigure 9b). Six additionalmagnetic intervals are obtained,yielding a completesequence of 14 intervalsincluding the Lower Catalan (Figure 10).

Discussion

Theeasterly directions found in Gamssteinand Mayerling arevery similar to directionspreviously obtained from early Jurassicsections from the NCA [e.g.,Mauritsch and Frisch, 1978,!980; Charmellet al., 1992].A largedifference in declinationis observed between the Mendlingbach and Gamssteinsections, although both sections belong to the sametectonic unit. But local and large rotations have already beenreported from the NCA [e.g.,Ga!let et al., 1996]. Moreover,the dataobtained from the Mendlingbach-East sectionshow a morecomplex tectonic history with the occurrenceof a rotationof approximately20 ø betweenthe Lowerand Upper Ladinian (Figure 11). The meandirections obtainedfrom the differentsections can thus be compared GALLETET AL.: MAGNETOSTRATIGRAPHY FROMNORTHERN CALCAREOUS ALPS 617

Conodonts Age Declination Inclination Magnetic 0 90 180 270 360 -90 -60 -30 0 30 60 90 zonation

Z < z

(

Z <

c• I _

(T3: 7m below)

Figure10. UpperLadin,an to LowerCarnian magnetostratigraphy fromthe Mayerlingsection. A sequenceof 14 magneticintervals is observed.Black bars within the lithostratigraphic column represent Partnach marl intercalations.

o 0 b) -EastMendlinc,(up•Lad.)•'• bach / ,•, Mendlingbach + \ a - o - Mendlin,,bach / %.--.+ Mendlinobach // _EaMendlingbach •t- (li5 •v•.•,•t-.) _•t•h•.•._. ß--lv•en¾9•s•~ -wt, /-East (1ovZ Lad.) + Mayerling 270•++ + + + + + + + +•I':+ + + + + • •0 270•+++ + + + + + + ++ + + ;•. + ++t •0 [ + •Men.dling•bac.h. ] teln • ++-East (up7Lad') / \ + •.'i X ++ Gamsstein

180 180 Figure11. Equal-areaprojection of the meandirections obtained h'om the Gainsstein, Mendlingbach and Mayerling sections(a) beforeand (b) after beddingcorrection. The dispersionof the inclinations notablydecreases after beddingcorrection. 618 GALLET ET AL.: MAGNETOSTRATIGRAPHY FROM NORTHERN CALCAREOUS ALPS

I tm!inf •;tm!ps•qoi•uo'-I 1I'•uo'-I

/ / / / / i•tmt. p.mqo•uo,-i I tm!p.mqol•uo'-I / / / / / / / / / / / / / / / / / / i / •;tm!p:mqoi•uo"I I tm!px•qolguo'-I ttm!tmsse.4 .-, / / / / - / / / •c> g/ L, / •.• [ NVI(I!:IVItODNO"I•; NV!(IklVItODNO"I t NVI(ItlVlitODNO"I •; NVINVSSVzI I NVINV$$Vd

/ /

ouoz !uoI•n D ouozs!suopoa, S Ouoz Z NVINV$SVzI I NVINVSSVd NVl•Aqql

I NVIOBVItODNO"I •; 'NVSSVd I tt••, , I

o 0 GALLETET AL.: MAGNETOSTRATiGRAPHY FROMNORTHERN CALCAREOUS 619

consideringonly the inclinations.The beddingcorrection in Figure!2 wealso report the magnetostratigraphic results clearlydecreases the dispersion of theinclinations (Figure 11; whichare presently available for theUpper Anisian and the in situ:lmean=53.3 ø, ct95=13.3ø, K=48.6, N=5; tilt corrected: Ladinian.From their compilation, Molina-Garza et al. [199I] Iracan=36.0ø,0t95=8.2 ø, K=126.4,N=5 [McFaddenand Reid, suggesteda Ladinian composite magnetic sequence which 1982]).The betterconsistency in inclinationafter bedding mostlydepends on the magnetostratigraphyobtained from a correction,together with a magnetizationessentially carried redbed section from Iberian Cordillera in eastern Spain [Turner by magnetiteand the presence of normaland reversed polarity et at., 1989]. This Ladiniancomposite contains seven directionsin all sections,indicate that the high-unblockingmagnetic polarity intervals with a predominantlynormal temperaturecomponents isolated from the three localitieswere magnetic polarity during the Lower Ladinian and a acquired during sediment deposition. In addition to this predominantlyreversed polarity during the Upper Ladinian. A primary component, our study shows evidence for a roughagreement can be suggestedfor theLower Ladinian, but remagnetizationphase in a reversedpolarity field. Becauseof no correlationis observedfor the UpperLadinian. Upper overlapswith other components,no reliablepalcomagnetic Anisian and Ladinian magnetostratigraphic results were also directioncan be estimatedfor this component.A similarcase obtainedby Muttoni eta!. [!994, !997] from the marine was recently observed from middle Triassic (Anisian) sedimentarysection of AghiaTriada (Hydra Island, Greece; limestonesfrom Albania[Muttoni et al., 1996]. Furthermore, Figure 12). Consideringthe new time calibrationconsidered a reversedpolarity componentof remagnetizationwas also byMuttoni et al. [1997],together with additional unpublished observedfrom the Langmoossection (close to Adnet; central biostratigraphicdata (L. Krystyn,1996), a verysatisfactory NCA [Galletet al., 1993b]).In thislatter study we suggesteda correlationis found for theFassanian 2 and the lower part of remagnefizationoccurring during the lastdeformation phase of the Longobardian1 (Figure 12). Age determinationsare much the NCA (i.e., during the late Eocene [Tollmann, !987; more difficultin the upperpart of the AghiaTriadia section Channell et al., !992]), although a very recent (Pantokratorlimestones), and the magnetostratigraphic remagnetizationpossibly during the Matuyama magnetic correlationwith the Austriancomposite sequence is poorly periodwas not excluded. constrained.Muttoni et aL [1997] alsorecently proposed an This studyprovides several magnetic polarity sequences upper Anisian to Ladinian magnetostratigraphyfrom the whichtogether document the Upper Anisian,the Ladinianand Fr/Stschbachsection (northern Italy; Figure 12). A coherent the Lower CamJan magnetostratigraphy.A comparison magnetostratigraphiccorrelation can be suggestedbetween between these sequencesis shown in Figure 12. The theselatter results, the Aghia Triadia sequence and the Austrian correlationsthat we propose(dashed lines) are basedon the data (dashedlines; Figure 12). The correlationof the interval conodontzonation and tephrochronology.A goodagreement Frli. ln from Fr/Stschbach with the interval C+ from the is obtainedbetween the results,although three thin, likely Austrian sequence is supported by field evidence in short, magnetic intervals are not confirmed within the FrOtschbachof a more reduced sedimentation rate close to the overlappingzones (they are indicated by half bars in the Fassanian1/2 boundary.Only the intervalFrln. lr, at the base compositesequence). We considerthat this is likely due to the of the Ladinian, cannot be correlated with the Austrian sampling(in fact, an effect resulting from the sampling sequence.But we remark that this short interval may be density,the duration of the magnetic intervals and the missing in Austria due to the slow sedimentationrate across stratigraphic completeness of the sections). Three other the Anisian-Ladinianboundary. intervalsare poorly definedby single samples(two in the Finally, it is possibleto roughly estimatethe magnetic Mendlingbach-Westsection and one in Gamsstein2). We reversal frequencyduring the Ladinian. Using the timescale proposea Ladinian compositemagnetic polarity sequence suggestedby Gradsteinet al. [I994] who proposeda duration which contains25 intervals, or ! 7 if the uncertain intervals of 6.9 m.y. for the Ladinian,a frequencyof 3.6 reversalsper are excluded(Figure 12). This compositesequence is shown million years is obtained if all the magnetic intervals are consideringan equal durationfor the biostratigraphiczones consideredand 2.5 reversalsper million yearsif the uncertain which should be modified when data on the relative duration of intervals are excluded. These mean values are similar to the theTriassic biostratigraphic zones will becomeavailable. Of estimateswe previouslyobtained for the Norian [GalIet et aL, course,these changes will not modify the numberand the age 1993a],but they are lower thanthe oneswe computedfrom the calibrationof the magneticintervals, but only their relative Carnian(between about 4.0 and4.8 reversalsper million years durationwithin the Ladinian.The Carniansequence obtained also considering Gradstein et al.'s timescale [Gallet et al., fromthe Mayerlingsection has alreadybeen discussed by 1994]). Galletet ai. [1994]. We recall the very goodagreement observedbetween this sequenceand the one previously Acknowledgments.We thankH. Th•veniautfor his helpduring the obtainedfrom Bolticektasi Tepe (southwestern Turkey [Gallet field work. We also thank S. Glider, H. Buther and J. Guex for comments. L.K. was financially supported by eta!., 1992]),although the samplingresolution in Mayerling Hochschuljubil•iumsstiftungder StadtWien (H-00234/94). This is !PGP istwice the one of Bo!ticektasiTepe. contribution1490 and DBT "Terreprofonde" contribution 9 I.

Figure12. Comparisonbetween the magnetostratigraphic sequences obtained from Gamsstein, Mendlingbach and Mayerling.Correlations between these sections are based on conodonts and tephrochronology (dashed lines). T1, T2 andT3 indicatethe location of threetuffaceous layers. A compositeLadinian magnetic polarity sequence is shown consideringanequal duration for thebiostratigraphic zones. Half bars correspond to poorly defined magnetic intervals. Wealso show for comparison the previous Ladinian composite sequence suggested byMolina-Garza et at. [1991]and themagnetostratigraphic resultsobtained from the pelagic limestone Aghia Triada and Fr6tschbach sections [Muttoni et al., 1994, 1997], 620 GALLET ET AL.: MAGNETOSTRATIGRAPHY FROM NORTHERN CALCAREOUS ALPS

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